Quality control of products can be an important aspect of manufacturing processes. A part of quality control in an industrial or manufacturing setting can include a visual inspection of a product that has been manufactured. In particular, visual inspection can relate to a visual inspection of a surface of a product. The objective of such a visual inspection is inter alia to check for possible defects or irregularities on a surface. If such defects are identified, and depending on the gravity of the defects, the product is normally to be discarded or repaired. The defects in this sense may be purely aesthetic but they can also be functional in that they would affect the use or operation of a product.
Different techniques are known for such a visual inspection of a surface to detect irregularities or defects. The techniques may be based on projecting a light onto a surface to be inspected and measuring a reflection of the light projected onto the surface. Depending on the type of surface, the reflection can be (or can expected to be) diffuse or specular. The angle between the normal of the surface and the light, and the angle between such normal and the camera are the usual parameters to be controlled.
A particular case is deflectometry. It is one such visual inspection technique that is used particularly for specular surfaces.
In general, deflectometry refers to procedures used to acquire topographical information on specular surfaces by the analysis of a reflected image of a known pattern projected onto the surface. A (potential) irregularity or defect is detected by comparing an expected reflection pattern with an actual reflection pattern. When the deviation of the actual reflection is above a predetermined threshold, a (potential) defect may be identified.
Deflectometry has been applied e.g. in the aviation and automotive industry. In these cases, rather large specular surfaces of relatively minor curvature are to be inspected. In order for deflectometry to give reliable results, it is important that the position and/or orientation of the source projecting the pattern onto the surface with respect to the area of the surface to be inspected is well controlled. If the projection with respect to the normal to the surface at the inspection point is not accurately controlled, the expected reflection cannot be well defined either. This can lead to both “false positives” (a potential defect is identified whereas in reality there is no such defect) and “false negatives” (a potential defect is not identified whereas in reality there is a defect).
The requirement of the precise determination of the position and orientation of the origin of the projection with respect to the normal to the surface becomes much more important when smaller objects with surfaces with significant curvature and/or doubly curved surfaces are to be inspected. It has been found that when known deflectometry techniques are used on more complexly curved surfaces, the results are unsatisfactory.
The need for a precise determination of position and/or orientation of the projection with respect to a normal to the surface at the inspection point is not specific for deflectometry, but is also important for other visual inspection techniques, particularly when the objects to be inspected become smaller and when they have more complexly curved surfaces.
The present disclosure provides examples of methods and systems for visual inspection that can improve visual inspection techniques.